Multiple EPROMs in one large switchable-bank EPROM?

Discussion in 'General Electronics Discussion' started by TTL, Jul 10, 2018.

  1. TTL

    TTL

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    I just came across this rather interesting photo in a forum dedicated to an 80's EPROM based digital drum machine:
    IMG_3438_2.JPG

    Alas, there's not a whole lot of information available ther than this:

    Since I have another EPROM based drum machine (Oberheim DMX) containing voice cards with either 2732 (4K) or 2764 (8K) EPROMs and I sometimes swap these with other EPROMs (for other sounds) I was wondering....

    Could an adapter board similar to the above be made, which plugs into the EPROM socket and contains a large EPROM of some sort which, by the help of DIP switches allow the voice card to see multiple 2732 or 2764 EPROMs? That would save me from swapping EPROMs all the time (a ZIF socket helps, but this would also eliminate having all those EPROMs around).
     
    TTL, Jul 10, 2018
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  2. TTL

    Harald Kapp Moderator Moderator

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    You could use a bigger EPROM (e.g. 27256) which has 4 times the capacity of a 2764. Tie the 2 most significant address bits (A14, A13) to pull-up resistors and a dip-switch to ground.
    Connect the other pins (address, data, enable, power) to the respetive pins of a 2764 compatible socket.
    Put the image of the first 2764 into the 1st 64 k of the 27256. Put the image of the 2nd 2764 into the 2nd 64 k of the 27256 etc.
    By setting the DIP switch you can select on out of 4 images.

    Use the three uppermost bits to emulate 2732 memory chips. 8 images of those will fit into one 27256.

    This is very likely the same method used in the circuit in your image using a 27512 chip and the 4 uppermost address bits to select one out of 16 images from 2732 chips.
     
    Harald Kapp, Jul 10, 2018
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  3. TTL

    TTL

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    Excellent!
    Here's a simple schematic I've made to see if I've understood correctly, with the different possibilities I have using different large EPROMs (27128, 27256 or 27512). The 2732 and 2764 pinouts are on the left (disregard the incomplete bus lines -the idea is to illustrate that the remaining address-lines, data-lines and power-lines join together):

    eprom_adapter.png

    So if I understand it correctly, with a 2732 I need to join together all the available address lines with the bigger EPROM (A0-A11) while the extra ones present only in the bigger EPROMs go to ground through a pullup resistor and the DIP switch?
    If that's correct, the same should apply for the 2764 which joins together A0-A12 between the 2764 socket and the bigger EPROM, while anything from A13 and beyond goes through a pullup resistor and a DIP switch, right?

    And I assume the D0-D7 pins are the same thing as O0-O7 which also join together between the 2732 (or 2764) socket and the bigger EPROM?
    Should CE (chip enable) as well as OE (output enable) also be connected between the appropriate socket pins and the larger EPROM pins?

    I think realistically I'll be using a 27256 in place of the 2732 socket and a 27512 in place of the 2764 socket which will allow for a total of 8 sound selections in both cases.
     
    Last edited: Jul 13, 2018
    TTL, Jul 13, 2018
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  4. TTL

    Harald Kapp Moderator Moderator

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    In theory yes, but that is not what your schematic shows. Your resistors are pull-down, the pull-ups are missing.
    upload_2018-7-13_13-40-48.png
     
    Harald Kapp, Jul 13, 2018
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  5. TTL

    TTL

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    You're absolutely right!
    I've corrected it here and also improved the schematics, first for the 2732, next for the 2764:
    2732_adapter.png
    2764_adapter.png

    Which values should the pullup resistors have?
     
    TTL, Jul 13, 2018
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  6. TTL

    Harald Kapp Moderator Moderator

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    That look good.
    The values for the pull-ups are absolutely uncritical. Anything from 330 Ω to 10 kΩ should work, whatever you find in your parts bin.
     
    Harald Kapp, Jul 14, 2018
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  7. TTL

    TTL

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    Thanks Harald.
    Another pleasant surprise I noticed by comparing datasheets is that the 2732 is pin-pin compatible with the 27256 by moving it down two pin-rows (except for one pin), so that should simpifly things.
    A quick comparing between the 2764 and 27512 (both 28 pin) also shows an almost all pin-pin compatibility.
    2732_27256 pinouts.png
     
    TTL, Jul 14, 2018
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  8. TTL

    TTL

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    A 27512 will most likely be used to replace the 2732, and a 27C010 for replacing the 2764. In both instances allowing 16 banks (i.e. 16 sounds) which means a 4-switch (16 position) DIP switch.

    I see that rotary DIP switches are available, which would be more practical IMHO. I assume they work in a similar manner as regular DIP switches.
    The datasheet in the above link lists different switch types: BCD, BCD-complement, hexadecimal, hexadecimal-complement. I think I saw some other types as well somewhere. Which type would be similar to the usual, rectangular DIP switches?
    2277719.jpg 38820.jpg
     
    Last edited: Aug 20, 2018
    TTL, Aug 20, 2018
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  9. TTL

    Harald Kapp Moderator Moderator

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    Electrically: yes.
    Mechanically: no.
    I presume that would be the hexadecimal type.
     
    Harald Kapp, Aug 20, 2018
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  10. TTL

    ratstar

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    Sorry for being a bit of a n00b. But I was wondering do those pull up resistors invert the data signal? (I could be completely off. I just never was told.) But they are just connected to switches, so maybe im wrong.

    But I know you can actually invert with a single pole, just is restrictive to continue doing it.
     
    ratstar, Aug 20, 2018
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  11. TTL

    Harald Kapp Moderator Moderator

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    No, they definitely do not. Look up e.g. this description.
     
    Harald Kapp, Aug 20, 2018
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